The present invention relates to a process for cleaning an electrically conducting surface, such as a metal surface.
Metals, notably steel in its many forms, usually need to be cleaned and/or protected from corrosion before being put to their final use. As produced, steel normally has a film of mill-scale (black oxide) on its surface which is not uniformly adherent and renders the underlying material liable to galvanic corrosion. The mill-scale must therefore be removed before the steel can be painted, coated or metallized (e.g. with zinc). The metal may also have other forms of contamination (known in the industry as "soil") on its surfaces including rust, oil or grease, pigmented drawing compounds, chips and cutting fluid, and polishing and buffing compounds. All of these must normally be removed. Even stainless steel may have an excess of mixed oxide on its surface which needs removal before subsequent use.
Traditional methods of cleaning metal surfaces include acid pickling (which is increasingly unacceptable because of the cost and environmental problems caused by the disposal of the spent acid); abrasive blasting; wet or dry tumbling; brushing; salt-bath descaling; alkaline descaling and acid cleaning. A multi-stage cleaning operation might, for example, involve (i) burning-off or solvent-removal of organic materials, (ii) sand- or shot-blasting to remove mill-scale and rust, and (iii) electrolytic cleaning as a final surface preparation. If the cleaned surface is to be given anti-corrosion protection by metallizing, painting or plastic coating, this must normally be done quickly to prevent renewed surface oxidation. Multi-stage treatment is effective but costly, both in terms of energy consumption and process time. Many of the conventional treatments are also environmentally undesirable.
Electrolytic methods of cleaning metal surfaces are frequently incorporated into processing lines such as those for galvanizing and plating steel strip and sheet. Common coatings include zinc, zinc alloy, tin, copper, nickel and chromium. Stand-alone electrolytic cleaning lines are also used to feed multiple downstream operations. Electrolytic cleaning (or "electro-cleaning") normally involves the use of an alkaline cleaning solution which forms the electrolyte while the workpiece may be either the anode or the cathode of the electrolytic cell, or else the polarity may be alternated. Such processes generally operate at low voltage (typically 3 to 12 Volts) and current densities from 1 to 15 Amps/dm.sup.2. Energy consumptions thus range from about 0.01 to 0.5 kWh/m.sup.2. Soil removal is effected by the generation of gas bubbles which lift the contaminant from the surface. When the surface of the workpiece is the cathode, the surface may not only be cleaned but also "activated", thereby giving any subsequent coating an improved adhesion. Electrolytic cleaning is not normally practicable for removing heavy scale, and this is done in a separate operation such as acid pickling and/or abrasive-blasting.
Conventional electrolytic cleaning and plating processes operate in a low-voltage regime in which the electrical current increases monotonically with the applied voltage (see FIG. 1 hereinafter at A). Under some conditions, as the voltage is raised, a point is reached at which instability occurs and the current begins to decrease with increasing voltage (see FIG. 1 hereinafter at B). The unstable regime marks the onset of electrical discharges at the surface of one or other of the electrodes. These discharges ("micro-arcs" or "micro-plasmas") occur across any suitable non-conducting layer present on the surface, such as a layer of gas or vapour. This is because the potential gradient in such regions is very high.